The present invention is directed to the art of spraying powder coating materials. More particularly, the invention is directed to a spray gun that is easy to clean internally and externally by substantially eliminating gaps and surfaces that can collect or trap powder.
Powder coating materials may be applied to any number of objects and surfaces by spraying. A commonly used spraying technique is electrostatic spraying with an electrostatic spray gun. In such a spraying apparatus, the spray gun typically includes a spray nozzle through which powder is ejected toward a target surface or object to be coated with the powder. Oftentimes, the object or surface is placed in a powder spray booth to constrain the powder within a confined area and to facilitate recovery of powder overspray.
Powder is fed to the gun from a powder supply, typically a powder feed hopper that may include a fluidized powder bed. The powder is fluidized in the hopper by a flow of air through the floor of the hopper. One or more powder pumps may be used to pump the fluidized powder from the hopper to one or more spray guns through a corresponding number of powder feed hoses. Such a powder spray apparatus is described in U.S. Pat. No. 5,454,256, which is assigned to the assignee of the present invention and is fully incorporated herein by reference. These are exemplary systems, however, and those skilled in the art will readily appreciate that the present invention can be used with a wide variety of powder spray apparatus.
Electrostatic powder spraying can be implemented in a number of ways. For purposes of the present invention, an electrostatic spray gun of particular interest is corona charging in which an electrostatic charge is applied to the powder being sprayed by exposing the powder to a corona or ion bombardment at the nozzle. This ion bombardment occurs when the electric field is high enough at the electrode to ionize air molecules. The electric field is produced by the electrode that is disposed at the nozzle and that is connected to a high voltage source, commonly referred to as a voltage multiplier. The target object or surface is held at an electrical potential relative to the electrode, typically ground, and the charged powder particles are attracted to and readily adhere to the target surface. Thus, a typical electrostatic corona charging powder spray gun includes an electrical power input cable, a powder hose and may further include an air line for purge air, all connectable to the back end of the spray gun.
A common problem with electrostatic spraying apparatus is the time and labor consuming task of color changeover. Powder coatings are characteristically made up of powder particles on the order of about thirty (30) microns in size, and in many cases can be substantially smaller. These small particles can easily find their way into various gaps and recesses within a spray gun housing, especially with the use of air pressure to force the powder through the gun housing and nozzle. In order to switch a gun from spraying a first powder color to another, as much of the first powder must be cleaned and removed from the gun as possible; otherwise, residual first powder color particles can mix with and contaminate the spray of the second powder color during subsequent use of the spray gun. It is also a common maintenance activity to clean a spray gun to remove excess powder from within the gun to prevent caking and clogging. Accordingly, it is typical for both routine maintenance and during color changeover to use air to blow off powder from various parts of the spray gun, both within the gun interior and that which may have collected on the gun exterior housing and supply lines.
Known electrostatic powder spray gun apparatus do not effectively prevent the entrapment or collection of powder within the gun assembly. This results in the time consuming and costly need to disassemble the gun in order to blow away the trapped powder and subsequent re-assembly of the gun components. Known gun apparatus also do not allow for gun purging with air through the powder path through the gun as part of routine maintenance and color changeover. Still further, the increasing use of spray booths for confining and recovering powder overspray has resulted in a need for better and easier gun mounting arrangements while still permitting fast and effective cleaning and color changeover.
Accordingly, it is an objective of the invention to provide a powder spray gun that can quickly and easily be cleaned both for maintenance and color changeover. Such a gun preferably will have minimal or negligible recesses or dead spots that can trap powder within the spray gun. Preferably, such a spray gun can also include an optional automatic gun purging function to assist in the cleaning operation. It is also an objective of the present invention to provide improved gun mounting arrangements while maintaining ease of assembly and color changeover and maintenance cleaning.
To the accomplishment of the foregoing objectives and others, the present invention provides in a first embodiment an electrostatic spray gun apparatus having a spray gun housing, a nozzle attached to a spray end of the housing, the nozzle having an electrode therein for electrostatically charging the powder, and a powder outlet through which powder is ejected towards a target surface to be powder sprayed, a powder supply or feed hose connectable to the housing at an inlet end thereof, and a powder path that extends in a substantially straight line along an axis of the housing from the powder inlet to the powder outlet. In accordance with one aspect of the invention, the powder path is realized in the form of an enclosed smooth powder passage that is substantially continuous and uninterrupted from the powder inlet to the powder outlet to eliminate substantially all recesses or gaps that could capture or trap powder. In a preferred form, the powder passage includes a plurality of tubular segments that are aligned along the housing axis and abut end to end. Still further preferred, these powder passage segments are held together in axial alignment by externally threaded connectors that when assembled in the housing axially compress the segments together to substantially eliminate dead spots or recesses to form the continuous smooth powder path.
In accordance with another aspect of the invention, a gun purge function is provided in the form of an adapter kit that allows a purge line to be installed on the gun assembly. This purge feature can alternatively be a standard feature of the gun, but as an optional feature it increases the flexibility of the gun design for the user. This gun purge feature assists in the cleaning and maintenance operations as well as facilitating color changeover. In accordance with a preferred embodiment of the purge function, the purge inlet connection is rotatable about the longitudinal axis of the gun housing in order to allow the purge inlet to be positioned so as not to interfere with other gun components.
In accordance with another aspect of the invention, with the use of a straight powder path, the spray gun voltage multiplier is mounted off axis with respect to the gun housing longitudinal axis. Accordingly, the multiplier is electrically connected to the gun electrode via a conductor that is angled toward the nozzle from the multiplier. In order to permit easy removal of the electrode for cleaning the gun interior, a conductor cartridge is provided between the gun electrode in the nozzle and the output of the voltage multiplier. In accordance with a further aspect of the invention, the conductor cartridge includes a valve, preferably in the form of a stem check valve, that closes when the gun electrode is removed or at least unseated from the nozzle. This valve when closed prevents powder from being blown into the gun housing and in particular toward the voltage multiplier. When open, the valve permits conventional air washed electrode operation.
In accordance with another aspect of the invention, improved gun mounting arrangements are provided. In one embodiment, a ball style bar mount is provided that permits the mounted gun to be oriented along two independent axes, for example, by rotating the gun about the vertical and horizontal axes. In another embodiment, the invention provides a tube mount arrangement in which an elongated mount tube extends from the rear of the spray gun to a mounting arrangement at the rear of the overall assembly. In a preferred form, the tube mount is rigidly held together with the gun housing in axial compression by a tie bar. This arrangement provides a very rigid and secure structure that will not loosen during vibration and normal spraying operations. Further, this arrangement facilitates fast and simple assembly and disassembly for repair and maintenance.
Various other embodiments of the invention are described and claimed herein, and other features and advantages of the present device will become apparent from the following detailed description, with reference to the accompanying drawings and claims, which form a part of the specification.